Using the keyword-only parameter should use the parameter name to force explicitly specify, such as configuring a class function or improving readability. When defining, use the position parameters and the keyword-only parameter, and the parameters must be passed in the keyword. Common uses include configuration options, backward compatibility, and improved readability. For example, in def greet(name, , greeting="Hello") greeting is a keyword-only parameter with default value. It must be written to greet("Alice", greeting="Hi") when calling. You can also add flexibility in combination with default values, such as format_data(data, *, sep=",", end="\n") to allow some parameters to be omitted. In actual development, it is used to control the interface call method to make the code clearer and safer.
The keyword-only parameter in Python is a very practical function when designing functions. It forces certain parameters to be passed only through keywords, not locations. This is especially useful when you want to specify parameters explicitly, avoid confusion, or improve code readability.

When should the keyword-only parameter be used?
The keyword-only parameter is suitable when you want the caller to have to explicitly write a parameter name. For example, in configuration functions, you may not want the user to confuse the parameter order, so you can set the key configuration item to keyword-only.
For example:
If you write a function send_email(to, subject, *, body)
, then you must write send_email("user@example.com", "Hi", body="Hello world")
when calling, otherwise an error will be reported. This prevents the body from being misrepresented as subject or other parameters.

How to define keyword-only parameters?
In Python, use an asterisk *
to separate positional parameters and keyword-only parameters. *
The latter parameters are all keyword-only.
def greet(name, *, greeting="Hello"): print(f"{greeting}, {name}")
In this example:

-
name
is a positional parameter. -
greeting
is a keyword-only parameter and cannot be passed through position.
So when calling:
- ? Correct:
greet("Alice", greeting="Hi")
- ? Error:
greet("Alice", "Hi")
will report an error
Note: You can also use
*
without adding any positional parameters to limit all subsequent parameters to be keyword-only.
Keyword-only is more flexible to use with default values
You can set default values for the keyword-only parameter so that they are not required, but still need to be passed in through the keyword.
For example, this function:
def format_data(data, *, sep=",", end="\n"): ...
The call method can be:
-
format_data([1,2,3])
→ Use default sep and end -
format_data([1,2,3], sep="|")
-
format_data([1,2,3], end="!")
This writing method makes the interface clear and prevents users from not remembering the parameter order.
Several common uses of keyword-only in actual development
- Configuration options : For example, a parser function accepts many optional configurations, using keyword-only can avoid confusion in the order of parameters.
- Backward compatibility : Set to keyword-only when adding new parameters to avoid breaking existing calls.
- Improve readability : When the parameter meaning is not very obvious, it will be clearer to force keyword transmission.
For example:
def create_user(username, password, *, is_admin=False, email=None): ...
In this way, you can tell whether it is an administrator account or an email address at a glance.
Basically that's it. Although the keyword-only parameter looks niche, it is very practical when writing complex functions or libraries, especially when you want to control the interface call method. If used well, it can make your code clearer and safer.
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